The invention relates to immersion lithography apparatus and methods, and particularly to apparatus and methods for recovering immersion fluid.
A typical lithography apparatus includes a radiation source, a projection optical system and a substrate stage to support and move a substrate to be imaged. A radiation-sensitive material, such as a resist, is coated onto the substrate surface before the substrate is placed on the substrate stage. During operation, radiation energy from the radiation source is used to project an image defined by an imaging element through the projection optical system onto the substrate. The projection optical system typically includes a plurality of lenses. The lens or optical element closest to the substrate can be referred to as the last or final optical element.
The projection area during exposure is typically much smaller than the surface of the substrate. The substrate therefore is moved relative to the projection optical system in order to pattern the entire surface of the substrate. In the semiconductor industry, two types of lithography apparatus are commonly used. With so-called “step-and-repeat” apparatus, the entire image pattern is projected at one moment in a single exposure onto a target area of the substrate. After the exposure, the substrate is moved or “stepped” in the X and/or Y direction(s) and a new target area is exposed. This step-and-repeat process is performed multiple times until the entire substrate surface is exposed. With scanning type lithography apparatus, the target area is exposed in a continuous or “scanning” motion. For example, when the image is projected by transmitting light through a reticle or mask, the reticle or mask is moved in one direction while the substrate is moved in either the same or the opposite direction during exposure of one target area. The substrate is then moved in the X and/or Y direction(s) to the next scanned target area. The process is repeated until all of the desired target areas on the substrate have been exposed.
Lithography apparatus are typically used to image or pattern semiconductor wafers and flat panel displays. The word “substrate” as used herein is intended to generically mean any workpiece that can be patterned including, but not limited to, semiconductor wafers and flat panel displays.
Immersion lithography is a technique that can enhance the resolution of lithography exposure apparatus by permitting exposure to take place with a numerical aperture (NA) that is greater than the NA that can be achieved in conventional “dry” lithography exposure apparatus having a similar optical system. By filling the space between the final optical element of the projection system and the resist-coated substrate, immersion lithography permits exposure with light that would otherwise be internally reflected at the optic-air interface. Numerical apertures as high as the index of the immersion fluid (or of the resist or lens material, whichever is least) are possible in immersion lithography systems. Liquid immersion also increases the substrate depth-of-focus, that is, the tolerable error in the vertical position of the substrate, by the index of the immersion fluid compared to a dry system having the same numerical aperture. Immersion lithography thus can provide resolution enhancement without actually decreasing the exposure light wavelength. Thus, unlike a shift in the exposure light wavelength, the use of immersion would not require the development of new light sources, optical materials (for the illumination and projection systems) or coatings, and can allow the use of the same or similar resists as conventional “dry” lithography at the same wavelength. In an immersion system in which only the final optical element of the projection system and its housing and the substrate (and perhaps portions of the stage as well) are in contact with the immersion fluid, much of the technology and design developed for dry lithography can carry over directly to immersion lithography.
However, because the substrate moves rapidly in a typical lithography system, the immersion liquid in the immersion area including the space between the projection system and the substrate tends to be carried away from the immersion area. If the immersion liquid escapes from the immersion area, that liquid can interfere with operation of other components of the lithography system. One way to recover the immersion liquid and prevent the immersion liquid from contaminating the immersion lithography system is described in US2006/0152697 A1, the disclosure of which is incorporated herein by reference in its entirety. Also see US2007/0222967 A1, the disclosure of which is incorporated herein by reference in its entirety.
The systems described in US2006/0152697 A1 and US2007/0222967 A1 include an immersion liquid confinement member. The immersion liquid confinement member includes an outlet through which immersion liquid is recovered (collected) from the immersion area. The outlet is covered by a liquid-permeable member such as a mesh or porous member. A vacuum control unit applies suction to a chamber associated with the outlet so as to draw the immersion liquid on the substrate through the liquid-permeable member and the outlet. It is important to control the suction force applied to the liquid-permeable member.